Overview
The president pro tempore of the United States Senate (often shortened to president pro tem) is the second-highest-ranking official of the United States Senate, after the vice president. According to Article One, Section Three of the United States Constitution, the vice president of the United States is the president of the Senate (despite not being a senator), and the Senate must choose a president pro tempore to act in the vice president's absence.
The president pro tempore is elected by the Senate as a whole, usually by a resolution which is adopted by unanimous consent without a formal vote. The Constitution does not specify who can serve in this position, but the Senate has always elected one of its current members. Unlike the vice president, the president pro tempore cannot cast a tie-breaking vote when the Senate is evenly divided. The president pro tempore has enjoyed many privileges and some limited powers.
During the vice president's absence, the president pro tempore is empowered to preside over Senate sessions. Except when necessary or to highlight important votes, the vice president and the president pro tempore rarely preside; instead, the duty of presiding officer is rotated among junior U.S. senators of the majority party to give them experience in parliamentary procedure.
Since 1890, the most senior U.S. senator in the majority party has generally (though not always) been chosen to be president pro tempore and holds the office continuously until the election of another. Since the enactment of the current Presidential Succession Act in 1947, the president pro tempore is third in the line of succession to the presidency, after the vice president and the speaker of the House of Representatives. The current president pro tempore of the U.S. Senate is Patty Murray of Washington. The most senior senator in the majority Senate Democratic Caucus and the first woman to hold the position, she was sworn in on January 3, 2023, at the start of the 118th Congress.
From Wikipedia (CC BY-SA 4.0).